Conventionally, error correction has been made possible by coding information signals and then decoding. Various methods are available for such an error correction. When the error correction is used for reliable wireless communications, a lower limit in a signal to noise ratio (Eb/No, where Eb is power per bit and No is noise power per 1 Hz), at which the error correction would be possible for information signals at a prescribed error rate, is a critical issue.
The lower limit in the signal to noise ratio, at which error correction would be possible, is theoretically determined by the Shannon's Theorem. Turbo coding is a method of error correction that offers a lower limit in the signal to noise ratio that is fairly close to a limit that would be determined by the Shannon's Theorem for error correction. The turbo coding was first introduced in a reference “C. Berrou, A. Glavieux, and G Montorsi; “New Shannon Limit Error Correcting Coding and Decoding: Turbo Codes (1),” Proc. of ICC'93 (Geneva, Switzerland), pp. 1064–1070”. The turbo coding has gained a lot of attention because of a significantly high performance.
Input signals for a turbo decoder must be normalized. A reference “C. Berrou, A. Glavieux and member, IEEE: “Near Optimum Error Correcting Coding and Decoding: Turbo-Codes,” Proc. of IEEE October, 1996, Vol. 44, No. 10, pp. 1261–1271” mentions a use of log-likelihood space ratios (LLR) for input signals to the turbo decoder. The LLR is defined as follows:
                              Λ          ⁡                      (                          d              κ                        )                          =                  log          ⁢                                          ⁢                                    P              ⁡                              (                                                      y                    κ                                    |                  1                                )                                                    P              ⁡                              (                                                      y                    κ                                    |                  0                                )                                                                        (        1        )            
Specifically, the LLR is defined as follows for an AWGN channel.
                              Λ          ⁡                      (                          d              κ                        )                          =                  4          ⁢                                          ⁢                                    E              s                                      N              o                                ⁢                                    y              κ                                                      E                s                                                                        (        2        )            
Here, Es is energy per symbol (1 bit after turbo coding); and No is noise power per 1 Hz. As shown in Equation 2, in order to obtain the LLR, No (noise power per 1 Hz) must be obtained. It means that a noise variance being added to a single piece of information must be estimated, and a receiving quality must be measured.
However, a size of a circuit for measuring the receiving quality would not be negligible in comparison to a size of a circuit for an entire decoder. In other words, the size of the circuit for the entire decoder would become larger.
If input signals to the turbo decoder were not to be normalized, then the use of the LLR can be avoided, and an enlargement of the circuit size can be inhibited. However, because trends in levels of the input signal into the turbo decoder could change significantly, it would be difficult to achieve a level of quality of the communications that would be of practical use, if the normalization were not to be performed.
A task of the present invention, therefore, is to provides, for example, a normalizing apparatus for normalizing the input signals to the turbo decoder without increasing the size of the circuit by too much.